Biophotonics

The term biophotonics denotes a combination of biology and photonics, with photonics being the science and technology of generation, manipulation, and detection of photons, quantum units of light. Photonics is related to electronics and photons. Photons play a central role in information technologies such as fiber optics the way electrons do in electronics.

Biophotonics can also be described as the "development and application of optical techniques, particularly imaging, to the study of biological molecules, cells and tissue". One of the main benefits of using optical techniques which make up biophotonics is that they preserve the integrity of the biological cells being examined.^[1][2]

Biophotonics has therefore become the established general term for all techniques that deal with the interaction between biological items and photons. This refers to emission, detection, absorption, reflection, modification, and creation of radiation from biomolecular, cells, tissues, organisms and biomaterials. Areas of application are life science, medicine, agriculture, and environmental science. Similar to the differentiation between "electric" and "electronics" a difference can be made between applications such as Therapy and surgery, which use light mainly to transfer energy, and applications such as diagnostics, which use light to excite matter and to transfer information back to the operator. In most cases the term biophotonics refers to the latter type of application.

Applications

Biophotonics can be used to study biological materials or materials with properties similar to biological material, i.e., scattering material, on a microscopic or macroscopic scale. On the microscopic scale common applications include microscopy and optical coherence tomography. On the macroscopic scale, the light is diffuse and applications commonly deal with diffuse optical imaging and tomography (DOI and DOT).

In microscopy, the development and refinement of the confocal microscope, the fluorescence microscope, and the total internal reflection fluorescence microscope all belong to the field of biophotonics.

The specimens that are imaged with microscopic techniques can also be manipulated by optical tweezers and laser micro-scalpels, which are further applications in the field of biophotonics.

DOT is a method used to reconstruct an internal anomaly inside a scattering material.^[3] The method is noninvasive and only requires the data collected at the boundaries. The typical procedure involves scanning a sample with a light source while collecting light that exits the boundaries. The collected light is then matched with a model, for example, the diffusion model, giving an optimization problem.

FRET

Fluorescence Resonance Energy Transfer, also known as Foerster Resonance Energy Transfer (FRET in both cases) is the term given to the process where two excited "fluorophores" pass energy one to the other non-radiatively (i.e., without exchanging a photon). By carefully selecting the excitation of these flurophores and detecting the emission, FRET has become one of the most widely used techniques in the field of Biophotonics, giving scientists the chance to investigate sub-cellular environments. See the full article on FRET

Light sources

The predominantly used light sources are beam lights. LEDs and superluminescent diodes also play an important role. Typical wavelengths used in biophotonics are between 600 nm (Visible) and 3000 nm (near IR).

Lasers

Lasers play an increasingly important role in biophotonics. Their unique intrinsic properties like precise wavelength selection, widest wavelength coverage, highest focusability and thus best spectral resolution, strong power densities and broad spectrum of excitation periods make them the most universal light tool for a wide spectrum of applications. As a consequence a variety of different laser technologies from a broad number of suppliers can be found in the market today.

Gas lasers

Major gas lasers used for biophotonics applications, and their most important wavelengths, are:

- Argon Ion laser: 457.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm (multi-line operation possible)

- Krypton Ion laser: 350.7 nm, 356.4 nm, 476.2 nm, 482.5 nm, 520.6 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm, 752.5 nm, 799.3 nm

- Helium–neon laser: 632.8 nm (543.5 nm, 594.1 nm, 611.9 nm)

- HeCd lasers: 325 nm, 442 nm

Other commercial gas lasers like carbon dioxide (CO2), carbon monoxide, nitrogen, oxygen, xenon-ion, excimer or metal vapor lasers have no or only very minor importance in biophotonics. Major advantage of gas lasers in biophotonics is their fixed wavelength, their perfect beam quality and their low linewidth/high coherence. Argon ion lasers can also operate in multi-line mode. Major disadvantage are high power consumption, generation of mechanical noise due to fan cooling and limited laser powers. Key suppliers are Coherent, CVI/Melles Griot, JDSU, Lasos, LTB and Newport/Spectra Physics.

Diode lasers

The most commonly integrated laser diodes, which are used for diode lasers in biophotonics are based either on GaN or GaAs semiconductor material. GaN covers a wavelength spectrum from 375 to 488 nm (commercial products at 515 have been announced recently) whereas GaAs covers a wavelength spectrum starting from 635 nm.

Most commonly used wavelengths from diode lasers in biophotonics are: 375, 405, 445, 473, 488, 515, 640, 643, 660, 675, 785 nm.

Laser Diodes are available in 4 classes:

- Single edge emitter/broad stripe/broad area

- Surface emitter/VCSEL

- Edge emitter/Ridge waveguide

- Grating stabilized (FDB, DBR, ECDL)

For biophotonic applications, the most commonly used laser diodes are edge emitting/ridge waveguide diodes, which are single transverse mode and can be optimized to an almost perfect TEM00 beam quality. Due to the small size of the resonator, digital modulation can be very fast (up to 500 MHz). Coherence length is low (typically < 1 mm) and the typical linewidth is in the nm-range. Typical power levels are around 100 mW (depending on wavelength and supplier). Key suppliers are: Coherent, Melles Griot, Omicron, Toptica, JDSU, Newport, Oxxius, Power Technology. Grating stabilized diode lasers either have an lithographical incorporated grating (DFB, DBR) or an external grating (ECDL). As a result, the coherence length will raise into the range of several meters, whereas the linewidth will drop well below picometers (pm). Biophotonic applications, which make use of this characteristics are Raman spectroscopy (requires linewidth below cm-1) and spectroscopic gas sensing.

Solid state lasers

Solid-state lasers are lasers based on solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions, or semiconductor lasers. (Although semiconductor lasers are of course also solid-state devices, they are often not included in the term solid-state lasers.) Ion-doped solid-state lasers (also sometimes called doped insulator lasers) can be made in the form of bulk lasers, fiber lasers, or other types of waveguide lasers. Solid-state lasers may generate output powers between a few milliwatts and (in high-power versions) many kilowatts.

Ultrachrome lasers

Many advanced applications in biophotonics require individually selectable light at multiple wavelengths. As a consequence a series of new laser technologies has been introduced, which currently looks for precise wording.

The most commonly used terminology are supercontinuum lasers, which emit visible light over a wide spectrum simultaneously. This light is then filtered e.g. via acousto-optic modulators (AOM, AOTF) into 1 or up to 8 different wavelengths. Typical suppliers for this technology was NKT Photonics or Fianium. Recently NKT Photonics bought Fianium,^[4] remaining the major supplier of the supercontinuum technology on the market.

In another approach (Toptica/iChrome) the supercontinuum is generated in the infra-red and then converted at a single selectable wavelength into the visible regime. This approach does not require AOTF's and has a background-free spectral purity.

Since both concepts have major importance for biophotonics the umbrella term "ultrachrome lasers" is often used.

Single photon sources

See single photon sources

Single photon sources are novel types of light sources distinct from coherent light sources (lasers) and thermal light sources (such as incandescent light bulbs and mercury-vapor lamps) that emit light as single particles or photons.

References